Laptop requirements for Blender
Blender should run okay on old hardware for some tinkering. I would only worry about buying a new computer if you experience performance issues with more complex models and rendering (especially Cycles). If you really need a lot of horse power, go for a desktop machine, laptop performance isn't ever great.

See the general specifications:
http://www.blender.org/download/requirements/

Dedicated Nvidia cards are recommended (CUDA support!), you should avoid ATI/AMD from what I've read (buggy OpenCL support etc.).